1. Field of the Invention
The present invention relates generally to topographic measurement of a target area using image sensors mounted on flying vehicles such as earth observation satellites.
2. Description of the Related Art
Topographic measurement using stereoscopic pictures is known as remote sensing technology. In one topographic measurement system known as a cross-track stereoscopic imaging system, a single image sensor (HRV and AVNIR sensors) mounted on a satellite (SPOT and ADEOS satellites) is used to capture a number of pictures of a target area at different times when the satellite is encircling on separate orbits. In other system known as an along-track imaging system, use is made of two image sensors (OPS and PRISM sensors) on board a single satellite (JERS-1 and ALOS satellites) to capture multiple pictures of a target area at different angles when the satellite is encircling on the same orbit over the target area. While the latter is able to send pictures at frequent intervals, hence available stereoscopic pictures can be easily obtained, a high-capacity memory is required on board the ship to store pictures before transmission to the earth. However, in applications where a high resolution of one meter is desired, the cross-track stereoscopic imaging has been preferred to the along-track stereoscopic imaging.
A recent advance in the remote sensing technology is the development of an earth observation satellite such as IKONOS and Quick Bird satellites in which a single sensor performs the functions of both cross-track imaging and along-track imaging systems.
When a pair of stereoscopic pictures is sensed, the pictures are scanned line by line and transmitted from the satellite in the form of frames to the earth station. The transmitted frames are analyzed in terms of point-to-point correlations between the frames to determine how much they differ from one another. This correlation information is known as parallax. Using a model of the image sensor, a position is determined in a three-dimensional coordinate system for each point-to-point correlation. A set of such 3-D position data obtained from a target area constitutes topographic data of the target area.
However, in order to sense a target area from an earth observation satellite, it is necessary to ensure that, when the satellite is approaching the target area, it is bright under sun light and not shadowed by any cloud. Chances for taking appropriate pictures are therefore limited. In particular, in applications where high resolution is desired, a single-sensor, cross-track earth observation satellite will be used. When the satellite is approaching a target area, the sensor must be pointed toward the target area from different angles at different times to obtain a pair of stereoscopic frames. Therefore, the target area must be clear and bright for both chances of image sensing. Additionally, the target area must be pointed from relatively large angles. This requires that the satellite orbits be distanced sufficiently from each other. During the time the satellite is encircling on intermediate orbits, no appropriate pictures cannot be taken, which leads to a low efficiency of satellite utilization. Therefore, target areas suitable for acquiring stereoscopic pictures are significantly limited.